A deflection-type differential refractive index detector for a liquid chromatograph has a quadrangular prism flow cell 100 constructed from two triangular prism chambers 100a and 100b as shown in FIG. 11A. The flow cell 100 has a partition plate 101 partitioning the two chambers 100a and 100b. 
A measurement beam B′ is irradiated on the flow cell 100 so as to sequentially transmit the two chambers of 100a for filling with or passage of the reference liquid Lc′ and 100b for filling with or passage of the a sample liquid Ls′. At this time, the flow cell 100 deflects the measurement beam B′ in accordance with the refractive index difference of the measurement beam B′ between the reference liquid Lc′ and the sample liquid Ls′, as shown in FIG. 11B. A change in the deflection angle is detected as the positional change of the measurement beam by a photodetector (non-illustrated) in a position apart from the flow cell 100 at a predetermined distance on the basis of the refractive index difference between the reference liquid Lc′ and the sample liquid Ls′ in the flow cell 100.
The deflection angle of the measurement beam B′ with the same refractive index difference is dependent on the angle between the optical axis of the measurement beam B′ and the partition plate 101, therefore the angle of the partition plate 101 is usually set to 45° to the optical axis of the measurement beam B′ so as to obtain a maximum sensitivity.
In the deflection-type differential refractive index detector, a sensitivity to the differential refractive index can be increased by extending the distance from the flow cell 100 to the photodetector. However, if the distance from the flow cell 100 to the photodetector is extended, not only drift increases due to distortion of the optical bench and the increase of temperature distribution, but also the device becomes too large. As a result, the method can not improve the overall performance. Namely, the method of extending the distance from the flow cell to the photodetector can not improve the signal/noise (S/N) ratio, even if improving the circuits of the photodetector.
A method of increasing the sensitivity by deflecting a beam twice has been proposed (e.g., see Patent references 1, 2) in the deflection-type differential refractive index detector.
For example, Patent Reference 1 described a construction in which two triangular prism chambers communicate with each other in the longitudinal side and a sample liquid flows from one chamber to the other chamber through the communication section. However, in the case of the construction described in Patent Reference 1, it needed more time to substitute the sample liquid in the chambers by flowing, because the flow passage is complicated, resulting in problem that a quick response to the inflow liquid change cannot be obtained. When a reference liquid flows or is sealed in a flow passage where a sample liquid flowed, or a sample liquid flows into a hollow prism where a reference liquid flowed or was sealed, there also is the problem that a quick substitution of a reference liquid cannot be made.
On the other hand, a deflection-type differential refractive index detector using two measurement beams has been described in Patent Reference 2. However, in the case of the construction described in the Patent Reference 2, it need more time to substitute the liquid in a section sandwiched by the two triangular prism chambers because they become a pentagonal prism (rough the shape of an M) with a large volume. Accordingly, when such a cell are used as a flow cell of a differential refractive index detector for liquid chromatography, there is the problem that they are not resistant to use from the viewpoint of responsiveness of the concentration change of a sample liquid and liquid replacement of a reference liquid.
Patent reference 1: Japanese Laid-Open Patent Application S46-2800 (FIG. 1)
Patent reference 2: Japanese Laid-Open Patent Application H3-170847 (FIG. 7)